The movie outputted open caption in Apple Proress 4444 with alpha always flicker

When I output a movie of open caption, in Apple Proress 4444 with alpha, the outputted movie flickers all the time.

But when I cancel the effect control of radial shadow , the outputted movie doesn't flicker any more.

How can I output a movie of open caption with radial shadow, in Apple Proress 4444 with alpha, but no flicker ？